In the prior art, patent document 1 proposes a machining center. In this machining center, a double column is arranged to extend laterally over a bed. A spindle head (spindle device) is mounted on the upper portion of the column in a manner that it is movable in the vertical direction and the lateral direction. The spindle head includes a spindle to which a tool is attached in a detachable manner. A table is arranged on the upper surface of the bed to support workpieces.
Patent document 2 proposes a vertical type machine tool. The vertical type machine tool includes upright side walls formed integrally with left and right sides of the bed. A cross-rail extends between the upper portions of the two side walls. A saddle is mounted on the cross-rail so as to be movable in a reciprocative manner in an X axis direction (lateral direction) and a Y axis direction (longitudinal direction). Further, a spindle head, which includes a spindle, is mounted on the saddle so as to be movable in a Z axis direction (vertical direction).
In the machining center described in patent document 1, the bed and the column form a base frame. The spindle head, a movement mechanism that moves the spindle head, and a rotary mechanism that rotates the spindle form a machining unit. In the machining center of patent document 1, the base frame and the machining unit are formed in an inseparable manner. This results in a shortcoming. That is, the base frame or the machining unit cannot be easily exchanged. Thus, when changing machine types, the entire machining center must be changed.
Like the machining center disclosed in patent document 1, the vertical type machine tool disclosed in patent document 2 is not formed so as to facilitate exchanging of the base frame or the machining unit and thus has the same shortcoming as the machining center of patent document 1.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 61-86144    [Patent Document 2] Japanese Laid-Open Patent Publication No. 1-146631